This invention concerns a measurement method, and particularly a method for measuring the gap between a multibladed rotatable member and a surrounding housing, and especially a method for measuring such gaps in a gas turbine engine.
This invention relates particularly to measuring the clearance between multibladed rotatable members and a surrounding housing in a gas turbine engine. The clearance gaps being measured could be any of the gap between a fan rotor path lining and a fan blade tip; the gap between a compressor seal segment and compressor blade tips; or the gap between a turbine seal segment and turbine blade fins.
During assembly and also maintenance of gas turbine engines such as used for example in aircraft, it is important to measure these clearances. Variations in clearance may be encountered due to wear and oxidation of particular segments, for instance in the lining of compressors and turbines. Also, the blades may be of different lengths and/or profiles.
Previously such clearances may have been measured by removing the respective rotor or rotors, and using a static or portable coordinate measuring machine, which is a quite time consuming operation. For instance, it may be necessary to remove a number of casings to permit removal of the rotor or rotors, and then to replace the casings to enable the measurement to be carried out. The process must then be repeated to allow the rotor to be replaced.
Not all coordinate measuring machines have a sufficient resolution to carry out such work. For instance, in a gas turbine jet engine the compressor requires an axial scan of the lining at sixteen circumferential positions on each stage of the rotor path, with an axial resolution of every 0.1 mm or better.
Fan tip clearances have been measured by measuring the gap directly and hand positioning the fan blade forwards and outwards, and measuring the gap between the blade and the casing.